Superior Wear and Corrosion Resistance with Hard Anodising

Whilst similar to sulphuric anodising, hard anodising produces a thicker oxide layer and so boasts increased corrosion and wear resistance. Hard anodising can usually produce thicknesses of 20µm to 70µm and up to 100µm on some aluminium alloys. Used for everything from saucepans to defence industry components, it’s the first choice for many applications wear a tough, hard-wearing exterior is crucial.

By using state-of-the-art equipment which allows us complete control of all process parameters, dimensions and geometry, we can maintain precise control over thickness whilst maintaining uniformity of the anodised finish. As specialist hard anodisers, Metal Supplies are able to achieve high coating thicknesses and wear resistance, even on historically difficult alloys.

As with sulphuric colour anodising, hard anodising can also be coloured by dyeing the anodic film though the colours are not as lustrous and aesthetically appealing because the additional thickness gives a duller finish.

Anodising is an electrolytic conversion coating, it transforms aluminium on the surface of components into aluminium oxide. Since aluminium converts rather than deposits, some of the thickness of the coating is ingress into the material and some of the thickness is growth out of the material. A good rule of thumb is that 50% of the anodic film is below the original surface and 50% is above. So a part that is anodised with a coating thickness of 10µm per surface would only have about 5µm of growth per surface.

So why opt for hard anodising?

Corrosion Resistance

Hard anodised aluminium demonstrates high corrosion resistance in harsh environments. Anodising will be vulnerable to alkalis and some acids, but our alloy anodising experts will be able to advise you on this and Metal Supplies are more than happy to provide compatibility testing.

Wear Resistance

Hard anodising is well-known for improving wear resistance. The best wear resistance is achieved at the higher thickness and by leaving the coating unsealed. Its wear resistance can be further improved by coating with polytetrafluoroethylene (PTFE).

Sealing

Hard anodising is common in both the sealed and unsealed states. For most applications, the most economic and most suitable sealing method is either cold sealing or hot water sealing. Dichromate sealing is heavily used in the aerospace and defence industries, but has a distinctive yellow/green colouration, which makes it unsuitable for parts where coating is cosmetic. Good sealing is imperative to corrosion resistance.

NOTE: Unsealed coatings are generally preferred when the hard anodising is for wear resistance. A suitable lubricant could be used to regain some of the corrosion resistance.

Which alloy?

Hard anodising gives best results on 6061 or 6082 alloys, but 6000 and 7000 series aluminium is generally suitable for hard anodising. Hard anodising is possible on 5000 series alloys, but properties such as wear resistance will be compromised. 1000 series are suitable for hard anodising, but may exhibit some crazing which is an expected part of the process.

It is unwise to use alloys with a high non-aluminium content if the coating is to be heated or cooled because the non-aluminium components of the alloy act as crack nucleation sites in the anodising. For this reason, it is ill-advised to use hard anodised 2000 series and 7075 for parts that will be subject shrink fitting (freeze fitting) or baked.

For more information on our hard anodising services please contact our friendly team on 0800 123 7010 or email: sales@metalsupplies.com